At what point is one needed? A certain weight or a certain trailer length?

Weight, not length. 500 pounds tongue (hitch) weight or more requires a WD hitch. And if you buy a cheap WD hitch that does not have built-in sway control you should be taken out at sunrise and shot.

Industry jargon is confusing. They say any trailer that grosses over 5,000 pounds or has a hitch weight of 500 pounds or more requires a WD hitch. But most TTs and other tandem-axle trailers have loaded hitch weight of 11.5% to 15%, so you will exceed 500 pounds hitch weight before you reach 5,000 pounds gross trailer weight. If a trailer has 15% hitch weight, then the max weight of any tandem-axle trailer you should tow with a weight-carrying hitch is about 3,333 pounds. So instead of the simple 500/5,000 max for hitch and trailer weight, I use a more accurate 500/3,333 max.

Go for a Reese Straight-Line WD hitch with dual-cam sway controls built in, or an Equali-I-Zer brand hitch which includes built-in friction type sway control. Or if you want the best sway control available, then go for a ProPride hitch.http://www.propridehitch.com/product...ol-Hitch-.html

To muddy the water a bit, you might want a sway controller even if your trailer doesn't have anywhere near 500 pounds hitch weight. In my case, I have a 5x10 golf cart trailer that I tow behind a 5er. The wet and loaded trailer weighs about 2,500 pounds, with hitch weight about 300 pounds (12% hitch weight). But when the 5er does a bobble, the second trailer does a huge sway. So I use an EZ-Lift friction sway control bar to control the sway on the golf cart trailer, even though I don't tow it with a WD hitch.

I find it really confusing when folks use the term "equalizer" for a WDH (weight distribution hitch) without any sway control. In fact I got burned once by a salesman who claimed not to know the difference between that and "Equal-I-Zer" which is a brand name for a fancy WDH with built in sway control.

Just saying ...

Mine is the Equal-i-zer, i was just too lazy to type it.

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this is all great info!!! just want to add, sway protection shouldn't be used as a band aide because you're experiencing sway, that's an issue with the way the trailer is loaded. you still want sway protection just not for that reason.

I've had the Equal-I-Zer (and one similar i think it's called e2?) and a very old version that was likely the per-cursor to the reese dual cam/straight line. the Equal-I-Zer was much much easier to dial in but the reese worked better IMO after it was dialed in. I believe friction bars (the bars you add to the side of a WDH) are generally for/work better for trailers that aren't that long and you aren't supposed to back up with them on.

if anyone has experience with the anderson WDH i'm interested... I've heard it puts more stress on the coupling part of the tongue and some people have had the stop break off and it lift the tongue off the ball. I'm nervous about that but it seems extremely easy to get it dialed in and it doesn't weigh much at all.

Why aren't you supposed to back up with the anti-sway.
I have one that looks like that but not Reese, and I leave it on until I am finished moving. Obviously you can't jack-knife too far.

Who says you can't back up?

that's the reason I've heard. if you bottom it out the travel of the friction bar or over extend it you can obviously do damage.

edit: i guess there is more http://www.etrailer.com/question-65717.html
but an equal-ilzer, dual cam, anderson, (i think all with build in sway) resist movement. I wonder if for some reason the friction bars resist movement more than the built in types when reversing?

that's the reason I've heard. if you bottom it out the travel of the friction bar or over extend it you can obviously do damage.

edit: i guess there is more http://www.etrailer.com/question-65717.html
but an equal-ilzer, dual cam, anderson, (i think all with build in sway) resist movement. I wonder if for some reason the friction bars resist movement more than the built in types when reversing?

All of those are friction based. But your reasoning is correct, some sway-control systems will run out of travel during tight reverse maneuvers.